Antenna configuration for co-operative beamforming

ABSTRACT

The present invention relates to a method for communicating in a network, the network comprising at least a first cell and a second cell including respectively a first primary station having a first antenna array dedicated to the first cell and a second primary station having a second antenna array dedicated to the second cell, for communicating with a plurality of secondary stations, the method comprising the step of (a) providing with a co-operative beamforming transmission from the first and second primary stations to at least one first secondary station, wherein step (a) includes (a1) the first secondary station signaling at least one channel matrix to at least one of the first and second primary stations, and (a2) the first and second primary stations applying a precoding matrix across both the first antenna array and the second antenna array, and wherein the precoding matrix comprises a first vector for the first cell and a second vector for the second cell, the precoding matrix being based on the at least one channel matrix.

FIELD OF THE INVENTION

The present invention relates to a method of communication in acommunication system like a mobile communication system, for exampleUMTS, LTE or LTE Advanced.

More specifically, the invention relates to a method of communicationusing cooperative beamforming, where the beamforming is obtained byusing primary station antennas from different cells, and wherepre-coding codebooks for co-operative beamforming between differentcells are used.

BACKGROUND OF THE INVENTION

In order to achieve greater transmission rate, it has been proposed touse a cooperative beamforming, where a plurality of antennas fromdifferent cells are used to create at least one spatial stream towards auser terminal. In connection with the beamforming and MIMO techniques, aprecoding is applied on the antennas by applying complex coefficients onthe antennas, so that the gain and/or the phase of the transmittingantennas are modified. It may be nevertheless crucial to signal to theuser terminal some information about the beamforming (e.g. so that theuser terminal can derive a suitable phase reference for receiving thesignal).

This can be done by means of predetermined codebooks, being a set ofpredetermined coefficients. An index of the selected codebook may besignalled for instance to the receiving station, so that a correspondingset of receiving coefficients is applied. Moreover, in order to providefeedback to the transmitting stations, the user terminal needs toindicate information about the channel, for example a preferredprecoding vector. It is thus required that all this signalling isimplemented in a clever way so that it does not create too muchoverhead, but it is still reliable and effective enough.

In systems such as UMTS and LTE, multi-antenna transmission/receptiontechniques variously described as, MIMO, precoding or beamforming aresupported for transmissions from a single cell to a mobile terminal.Precoding codebooks are defined, which enable the User Equipment (UE) toreport a preferred precoding index for downlink transmission.

The same codebook may be used to signal to a user equipment theprecoding vector or matrix which is actually applied in the downlink bya base station. This may be viewed as a way of describing the channelcoefficients. This enables the user equipment to derive an appropriatephase/amplitude reference signal from common pilot symbols fordemodulation of downlink transmissions. Alternatively the pilot symbolsmay be precoded to form dedicated reference symbols and used directly asreceiver phase reference.

Typically, the specification for systems like LTE makes use of the term“antenna port”, which is effectively a virtual antenna which may bederived by a linear combination of signals from one or more physicalantennas. For convenience, we use the term “antenna”, but this couldalso be understood as “antenna port”

Multi-antenna techniques such as beamforming, using antennas frommultiple cells or multiple sites, could be of interest in LTE. However,there is a need of specifying how codebooks should be defined for suchcases so that the signalling may be simple but still effective.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide with a method forproviding codebooks that could be used for cooperative beamforming.

It is another object of the invention to provide with a method ofsignalling the precoding in a cooperative beamforming context, where aplurality of cells are cooperating.

In accordance with a first aspect of the invention, a method is proposedfor communicating in a network, the network comprising at least a firstcell and a second cell including respectively a first primary stationhaving a first antenna array dedicated to the first cell and a secondprimary station having a second antenna array dedicated to the secondcell, for communicating with a plurality of secondary stations, themethod comprising the step of

(a) providing with a co-operative beamforming transmission from thefirst and second primary stations to at least one first secondarystation, wherein step (a) includes

(a1) the first secondary station signaling at least one channel matrixto at least one of the first and second primary stations, and

(a2) the first and second primary stations applying a precoding matrixacross both the first antenna array and the second antenna array, andwherein the precoding matrix comprises a first vector for the first celland a second vector for the second cell, the precoding matrix beingbased on the at least one channel matrix.

Thus, the precoding can be carried out and signalled with a precodingdesigned for single cell precoding. Since in an embodiment of theinvention, the system may implement single cell MIMO along with themulti cell MIMO (or co-operative MIMO), this avoids using a specialsignalling, and enables to use the single cell signalling for bothtransmission modes.

In accordance with a second aspect of the invention, a first primarystation is proposed having a first antenna array dedicated to a firstcell of a network, the network further comprising at least a second cellincluding a second primary station having a second antenna arraydedicated to the second cell, the first primary station comprising meansfor communicating with a plurality of secondary stations, the primarystation further comprising means for collaborating with the secondprimary station for providing with a co-operative beamformingtransmission from the first and second primary stations to at least onefirst secondary station, receiving means for receiving from the firstsecondary station a signaling of at least one channel matrix, andcontrol means for applying a precoding matrix across the first antennaarray, and wherein the precoding matrix comprises a first vector for thefirst cell and a second vector for the second cell, the precoding matrixbeing based on the at least one channel matrix.

In accordance with a third aspect of the invention, a secondary stationis proposed comprising means for communicating in a network, the networkcomprising at least a first cell and a second cell includingrespectively a first primary station having a first antenna arraydedicated to the first cell and a second primary station having a secondantenna array dedicated to the second cell, for communicating with thesecondary station, the secondary station comprising means for receivinga co-operative beamforming transmission from the first and secondprimary stations to at least one first secondary station, and forsignaling at least one channel matrix to at least one of the first andsecond primary stations, the matrix being based on a codebook dedicatedfor the first cell and/or a codebook dedicated for the second cell.

In accordance with a fourth aspect of the invention, it is proposed anetwork comprising at least two primary station of the second aspect andat least one secondary station of the third aspect.

These and other aspects of the invention will be apparent from and willbe elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail, by way ofexample, with reference to the accompanying drawings, wherein:

FIG. 1 is a block diagram representing a network in which is implementeda first embodiment of the invention;

FIG. 2 is a block diagram representing a network in which is implementeda first embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a cellular network comprising aplurality of cells. In each cell, a dedicated primary station, like aneNodeB in LTE, communicates with mobile terminals, like User Equipments,located within the considered cell. Each primary station may comprise aplurality of antennas dedicated to the considered cell, the antennasbeing arranged in an array. In order to carry out beamforming or MIMOcommunication, the gain/phase of each antenna is modified when theprimary station is transmitting. This is called precoding, and whichpermits to create spatial beams or spatial data streams directed towardsa predetermined secondary station for example.

In order to control the precoding, feedback may be needed from thesecondary station. For example, channel quality is typically indicatedby a channel quality indicator (CQI). This is typically indicates a datarate at which packet transmissions can be received with pre-determinederror probability.

Multi-antenna techniques such as beamforming, using antennas frommultiple cells or multiple sites, are discussed (under the descriptionof CoMP or Co-operative Multi-Point transmission). In such a case, aplurality of cells (typically two cells but could be more) arecollaborating for creating a beam to one or more secondary stations. Forinstance, the same data streams are transmitted from antenna ports ofdifferent cells.

However, it is not clear how codebooks should be defined for such cases,and it is not clear how existing (or new) codebooks designed for singlecell use should be adapted for multi-cell use. An additional problem isthat if co-operative beamforming is applied to transmit data frommultiple cells, then copies of the data are needed at each of thecooperating cells.

Therefore, it is investigated in accordance with the embodiments of thisinvention to provide with solutions for a simple and reliable signallingand precoding that could be used in co-operative multicell MIMOcommunications. In a first variant of these embodiments, it can beproposed to use codebooks designed for single cells to report thechannel information for multiple cells, where a phase offset isindicated between pairs of codebook indices. Moreover, this could aswell be still improved with the transmission from multiple cells to amobile terminal based on the reported channel information whereindependent data streams are sent from different cells. This could bedone under the assumption of the use of this method, and in such a case,the mobile terminal can compute a channel quality indication (CQI) foreach stream. It is to be noted that these features might be implementedindependently.

In a first network illustrated on FIG. 1, a first cell 101 and a secondcell 102 are neighboring. A base station 200 may comprise a firstprimary station 201 which antenna ports 211 are dedicated to the firstcell 101 and a second primary station 202 which antenna ports 212 arededicated to the second cell 102. In such a configuration, the same datacan be easily transmitted by the first antenna ports 211 and the secondantenna ports 212. A spatial stream 301 from the two cells is directedtowards a secondary station 401 located in the first cell. Moreover, itis possible as shown to have another spatial stream 302 directed toanother secondary station 402. In the illustrated example, the secondarystation 402 is in the second cell, however, it could be located in thefirst cell as well.

In a first embodiment in a system like LTE, co-operative beamforming maybe carried out between two cells in the downlink like illustrated inFIG. 1. The precoding is assumed to be carried out for each spatialstream by applying a precoding vector across the set of antennasbelonging to both cells. The precoding vector is composed of a vectorfor each cell which is selected from a set of predetermined codebooks(one codebook for each of the first cell 101 and the second cell 102).The precoding is signalled for a spatial stream by indicating theprecoding vector for each cell. An individual user, like the secondarystations 401 or 402 may receive one or more spatial streams 301 or 302respectively. In order for the network to select appropriate precodingvectors, the mobile terminal provides feedback on the state of thechannel(s). The feedback may take the form of a set of indices topreferred precoding vectors for each stream and each cell. These vectorsare chosen to maximize the possible total transmission rate. Thecodebook(s) at the mobile terminal are the same as those in the network.

In a variation of this first embodiment, the codebooks are designed sothat the coefficients applied to each antenna are defined with respectto the coefficients applied to one of the antennas, which acts as areference. In LTE the first antenna is the reference and this antennahas a coefficient of unity. In order to fully define the requiredprecoding across multiple cells using multiple codebooks (one for eachcell) it may be necessary to specify the required phase/amplituderelationship between the codebooks. For LTE the codebook elements haveunit magnitude, so the phase relationship could be specified as a phaserotation between one codebook and another (e.g. as a phase rotationbetween successive codebooks or with respect to a reference codebook).This phase offset could be specified with 2 or 3 bits to indicate one of4 or 8 different phase offsets. In general, the phase value may besignalled in the uplink from the mobile terminal. In order to define aphase reference for receiving a signal from multiple cells, the phaseoffset can also be signalled in the downlink. It is to be noted that inthe network illustrated on FIG. 1, the first primary station 201 and thesecond primary station 202 being in the same device, this permits tosimplify the management of the phase offset, since this can be known bythe base station 200. The use of the phase offset is howeveradvantageous, as it facilitates the use of existing codebooks designedfor single cell use. In an example of this embodiment, the referencecodebook is chosen as the primary station in which is located the userequipment. As a consequence, the reference codebook for the secondarystation 401 would be cell 101's codebook and the reference for thesecondary station 402 would be cell 102's codebook.

In a variant of the first embodiment, the network can be somewhatdifferent as illustrated on FIG. 2. In this case, the first primarystation 201 of the first cell 101 may be in a different device than thesecond primary station 202 of the second cell 102. Thus, it is a bitmore difficult to synchronise and to transmit the same datasimultaneously. Moreover, the estimation of the phase offset may be moredifficult than in a network illustrated on FIG. 1. However, it is stillpossible to implement the first embodiment in such a network, especiallyin the case of a femto-cells for example.

It is however to be noted that the network of FIG. 2 is more adapted tothe implementation of the second embodiment of the invention. Inaccordance, with the second embodiment, the precoding in the primarystations comprises applying a precoding matrix across the first antennaarray or the second antenna array. Moreover, the precoding matrixcomprises a first vector for the first cell or a second vector for thesecond cell.

In the second embodiment implemented in a system like LTE, co-operativetransmission may be carried out between two cells in the downlink. Theprecoding is assumed to be carried out for each spatial stream byapplying a precoding vector across the set of antennas belonging to onlyone cell. The precoding vector for each cell is selected from a set ofpredetermined codebooks (one codebook for cell). The precoding issignalled for a spatial stream by indicating the precoding vector foreach cell. An individual user may receive one or more spatial streamsfrom each cell. In order for the network to select appropriate precodingvectors, the mobile terminal provides feedback on the state of thechannel(s). The feedback takes to form of a set of indices to preferredprecoding vectors for each stream and each cell. The set of precodingvectors are chosen to maximize the possible total transmission rate. Thecodebook(s) at the mobile terminal are the same as those in the network.

The second embodiment has the following advantages:

-   -   Different data can be transmitted from the different cells,        which avoids the need to provide multiple copies of the same        data in several cells.    -   Existing codebooks can be used without the need to indicate any        phase offset    -   Scheduling decisions (e.g. transmission rate and power        allocation) can be at least partly independent across the        different cells. That is the reason why this second embodiment        may be more adapted to the network illustrated on FIG. 2.

As an additional feature to the second embodiment, the signalling toindicate the downlink transmission of multiple streams to a userequipment from multiple cells could be similar to that used for multiplestreams from a single cell (i.e. PDCCH in LTE). In general an indicationwould be needed to the user equipment of which was the appropriatereference signal to receive a given stream. If the reference is providedby means of precoded reference symbols, then it may not be necessary toindicate from which cell the stream is transmitted, but only whichreference symbols (or reference sequence) should be used.

Moreover, some variants that could apply to the first embodiment and thesecond embodiment may comprise the following.

The phase reference(s) is indicated in the downlink using precodedreference symbols or precoded pilot symbols. Thus, the reference symbolsmay be used to carry information, and are not just used for the sake ofsynchronisation or likewise.

Moreover, it is to be noted that in the description, the expression ofprecoding vectors was used, although in some variant of the invention,the precoding can be represented with the help of precoding matrices.

As an additional feature, the user equipment computes the channelquality indicator (CQI) for each spatial stream, under the assumption ofa selected number of spatial streams, and under the assumption of theuse of a selected codebook entry for each cell. The CQI is signalled tothe network and it enables the network to select an appropriatetransmission rate (e.g. modulation and coding scheme).

Additional feedback may be provided to indicate alternative sets ofprecoding vectors (and CQI) which do not necessarily maximise the totalrate. For example there could be a set of feedback computed for eachpossible number of spatial streams. Feedback could be provided fordifferent assumptions about the interference from other transmissions toother terminals (e.g. in the second embodiment, that the cells are bothsending streams to the same user equipment, or one cell is nottransmitting, or the cells not co-operating in which there would beinterference).

It is to be noted that the feedback could comprise a quantizedrepresentation of the channel instead of an index to a precoding vector.

Moreover, the feedback may indicate preferred transmission rank (whichis in fact a preferred number of spatial streams).

In neither embodiment is it necessary that the number of antennas is thesame in the co-operating cells.

The invention is applicable to systems using co-operative beamformingbetween cells which may include LTE-Advanced. The cells may be located asingle base station site, or on different sites, for example femto-cellsimplemented by fibre radio techniques.

In the present specification and claims the word “a” or “an” precedingan element does not exclude the presence of a plurality of suchelements. Further, the word “comprising” does not exclude the presenceof other elements or steps than those listed.

The inclusion of reference signs in parentheses in the claims isintended to aid understanding and is not intended to be limiting.

From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in the art of radiocommunication.

1. A method for communicating in a network, the network comprising atleast a first cell and a second cell including respectively a firstprimary station having a first antenna array dedicated to the first celland a second primary station having a second antenna array dedicated tothe second cell, for communicating with a plurality of secondarystations, the method comprising the step of (a) providing with aco-operative beamforming transmission from the first and second primarystations to at least one first secondary station, wherein step (a)includes (a1) the first secondary station signaling at least one channelmatrix to at least one of the first and second primary stations, and(a2) the first and second primary stations applying a precoding matrixacross both the first antenna array and the second antenna array, andwherein the precoding matrix comprises a first vector for the first celland a second vector for the second cell, the precoding matrix beingbased on the at least one channel matrix.
 2. The method of claim 1,wherein the first vector and the second vector are selected respectivelyfrom a first codebook dedicated to the first cell and from a secondcodebook dedicated for the second cell.
 3. The method of claim 1,wherein step (a1) comprises signaling a phase relationship between thefirst vector and a second vector.
 4. The method of claim 1, wherein aphase reference is signaled to a secondary station by means of aprecoded reference symbol.
 5. The method of claim 1, wherein theprecoding matrix is a vector.
 6. The method of claim 1, wherein thefirst secondary station signals for each spatial stream from the firstcell and/or the second cell a channel quality information.
 7. The methodof claim 6, wherein the channel quality information further comprises atleast one alternative preferred precoding vector, and wherein the firstprimary station and/or the second primary station selects either thepreferred precoding vector or the alternative vector depending on aninterference measurement.
 8. The method of claim 7, wherein the channelquality information is representative of the channel conditions.
 9. Themethod of claim 6, wherein the channel quality information includes apreferred transmission rank.
 10. The method of claim 1, wherein the atleast one channel matrix comprises one of a preferred precoding vectorselected by the secondary station from a codebook; a quantizedrepresentation of the transmission channel between one of the first andsecond primary stations and the secondary station.
 11. The method ofclaim 12 wherein the preferred precoding vector is selected to indicateone of: the precoding vector which would give the highest received datarate; the precoding vector which would give the highest received SNR;when used for transmission by one of the first or second primarystations.
 12. A first primary station having a first antenna arraydedicated to a first cell of a network, the network further comprisingat least a second cell including a second primary station having asecond antenna array dedicated to the second cell, the first primarystation comprising means for communicating with a plurality of secondarystations, the primary station further comprising means for collaboratingwith the second primary station for providing with a co-operativebeamforming transmission from the first and second primary stations toat least one first secondary station, receiving means for receiving fromthe first secondary station a signaling of at least one channel matrix,and control means for applying a precoding matrix across the firstantenna array, and wherein the precoding matrix comprises a first vectorfor the first cell and a second vector for the second cell, theprecoding matrix being based on the at least one channel matrix.
 13. Asecondary station comprising means for communicating in a network, thenetwork comprising at least a first cell and a second cell includingrespectively a first primary station having a first antenna arraydedicated to the first cell and a second primary station having a secondantenna array dedicated to the second cell, for communicating with thesecondary station, the secondary station comprising means for receivinga co-operative beamforming transmission from the first and secondprimary stations, to at least one first secondary station, and forsignaling at least one channel matrix to at least one of the first andsecond primary stations, the channel matrix being based on a codebookdedicated for the first cell and/or a codebook dedicated for the secondcell.
 14. A network comprising at least a first primary stationdedicated to a first cell, and a second primary station dedicated to asecond cell, the first primary station and second primary stationcomprising means for collaborating for providing with a co-operativebeamforming transmission directed to a secondary station a, thesecondary station comprising means for communicating in a network, thenetwork comprising at least a first cell and a second cell includingrespectively a first primary station having a first antenna arraydedicated to the first cell and a second primary station having a secondantenna array dedicated to the second cell, for communicating with thesecondary station, the secondary station comprising means for receivinga co-operative beamforming transmission from the first and secondprimary stations to at least one first secondary station, and forsignaling at least one channel matrix to at least one of the first andsecond primary stations, the channel matrix being based on a codebookdedicated for the first cell and/or a codebook dedicated for the secondcell.